“The Russians kicked out a whole bunch of our people,” John Sipher, a 20-year CIA veteran, told the Times. “Our station in Moscow is probably really small now and they are under incredible surveillance.”

The Kremlin, he added, had shown increasing willingness to silence critics and informants.

“The Russians are very focused and upset,” he added. “They have shown they are willing to kill sources.”

Russian election interference efforts in 2016 were exposed in the months following the election of Trump, who has repeatedly cast doubt on Moscow's involvement.

National security adviser John Bolton said that he warned his Russian counterpart Thursday during a meeting about the dangers of meddling in future U.S. political activities.

"I made it clear that we wouldn’t tolerate meddling in 2018, and that we were prepared to take necessary steps to prevent it from happening," Bolton said.